This invention relates to phase-locked fiber lasers that provide diffraction limited, high brightness and/or linearly polarized output laser beams, employing modal discriminating multiple cores and/or single or multiple oblong cores.
If the cores of a multi-core fiber laser array are configured in a hexagonal series of isometric rings, the evanescent-wave coupling among all adjacent emitters provides spontaneous phase locking so as to combine the power of all supermodes coherently into a single bright beam centered on the fiber axis, as is described in U.S. Pat. No. 6,031,850. Because a very strong evanescent-wave interaction over a very long fiber length, the double-clad multi-core fiber laser array is naturally phase-locked to emit all supermodes co-existing in the structure. In the case of a single isometric ring of six cores surrounding a central core, there exists five supermodes. In a large multicore phase-locked fiber laser system containing many isometric rings, there exists a very large number of supermodes, which can all oscillate within certain spectral or frequency locking range, when the pump power exceeds the lasing thresholds of these modes. Two methods commonly used to select a preferred mode from a multimode system are regulating the cavity width and spatially filtering out the unwanted modes. Both of these methods are useful only for low power lasers; for high power lasers, they are neither effective nor practical because a very high penalty must be paid by trading off output power for modal purity.
Prior art fiber lasers having multiple cores arranged in one or two isometric rings and doped with rare-earth ions can produce hundreds of thousands of watts of output power. The high-brightness beam size is typically on the order of 10 microns; therefore, the power density can be as high as 109 W/cm2, which far exceeds the power damage threshold of doped glass.
An important characteristic of multicore, phase-locked fiber lasers configured in an isometric geometry is that the polarization of the laser output is highly random. For many applications, a linearly polarized laser beam is desired. Use of polarization filters necessarily results in severe power reduction and loss of efficiency.
Objects of the invention include provision of a multicore phase-locked fiber laser array in which the fundamental, in-phase supermode is dominant, thereby emitting output power in a high-brightness and diffraction-limited laser beam; provision of a linearly polarized laser output from a clad-pumped, phase-locked fiber laser array; and provision of high laser power without high power densities which could damage the rare earth-doped fiber.
This invention is predicated in part on the discovery that it is very difficult or impossible to eliminate all the unwanted supermodes naturally existing in an isometric structure having equal mode-field coupling characteristics, under uniform index of refraction, gain, or dimension; therefore, the output power is distributed nearly equally among all allowed supermodes in a phase-locked fiber laser. Nonetheless, the output power can be altered by using a graded modal discriminating characteristic having the effect of increasing coupling toward the center of the array to thereby concentrate 90% or more of the power in a single, in-phase fundamental supermode at the center of the laser array. This invention is further predicated on the discovery that, although prior clad pumped single mode fiber lasers with circular cores provide output laser beams in which the polarization is highly randomly oriented, nonetheless, use of a non-circular core can remove the two-fold degeneracy of the HE11 mode of a cylindrical fiber waveguide, thereby establishing a linearly polarized output.
According to the present invention, a plurality of cores doped with a rare earth lasing ion, within a common pump cladding, includes a central core and additional cores disposed outwardly of said central core, and having either or both of (a) an oblong cross section, thereby to provide a linearly polarized output laser beam, or (b) a mode discriminating characteristic, such as index of refraction, gain or cross sectional dimension, which is graded and being lower for the outward cores than the central cores, thereby to transfer laser power coherently into a bright laser beam of the fundamental in-phase supermode. According further to the invention, instead of an array of oblong cores, a single oblong core in a very large cladding may be utilized to provide a linearly polarized output beam. According to the invention further, utilization of a graded modal discrimination characteristic provides the ability to separate the cores by as much as five-fold more than in the prior art, such as with center-to-center spacings of 15 to 50 microns, whereby the laser power in a single bright beam can be spread over a greater area and thereby provide very high power without prohibitively high power densities.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.